The present invention relates to cementing operations and, more particularly, in certain embodiments, to acid-soluble cement compositions that comprise cement kiln dust (“CKD”) and/or a natural pozzolan and associated methods of use.
Cement compositions may be used in a variety of subterranean applications. For example, in subterranean well construction, a pipe string (e.g., casing, liners, expandable tubulars, etc.) may be run into a well bore and cemented in place. The process of cementing the pipe string in place is commonly referred to as “primary cementing.” In a typical primary cementing method, a cement composition may be pumped into an annulus between the walls of the well bore and the exterior surface of the pipe string disposed therein. The cement composition may set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement (i.e., a cement sheath) that may support and position the pipe string in the well bore and may bond the exterior surface of the pipe string to the subterranean formation. Among other things, the cement sheath surrounding the pipe string functions to prevent the migration of fluids in the annulus, as well as protecting the pipe string from corrosion. Cement compositions also may be used in remedial cementing methods, for example, to seal cracks or holes in pipe strings or cement sheaths, to seal highly permeable formation zones or fractures, to place a cement plug, and the like. Cement compositions also may be used in surface applications, for example, construction cementing.
In some applications, it may be desirable for the cement composition to be acid soluble. For instance, an acid-soluble cement composition may be desirable in applications where it is anticipated that the hardened cement will be removed in subsequent well bore operations. One particular application includes use of an acid-soluble cement composition to plug permeable zones in a formation that may allow the undesired flow of fluid into, or from, the well bore. For example, the permeable zones may result in the loss of circulation of fluids, such as a drilling fluid or a cement composition, in the well bore or an undesired influx of gas or water into the well bore. The permeable zones include, for example, vugs, voids, fractures (natural or otherwise produced) and the like. Other applications for acid-soluble cement compositions include, for example, the formation of annular plugs and isolation of gravel-packed well bore intervals. Examples of acid-soluble cement compositions include those comprising Sorel cements and Portland cements.